Endothelial progenitor obsolescence and atherosclerotic inflammation

Endothelial progenitor cells in atherosclerosis

Endothelial progenitor cells (EPCs) are involved in the maintenance of endothelial homoeostasis and in the process of new vessel formation. Experimental and clinical studies have shown that atherosclerosis is associated with reduced numbers and dysfunction of EPCs; and that medications alone are able to partially reverse the impairment of EPCs in patients with atherosclerosis. Therefore, novel EPC-based therapies may provide enhancement in restoring EPCs' population and improvement of vascular function. Here, for a better understanding of the molecular mechanisms underlying EPC impairment in atherosclerosis, we provide a comprehensive overview on EPC characteristics, phenotypes, and the signaling pathways underlying EPC impairment in atherosclerosis.

Decreased number of circulating endothelial progenitor cells in patients with Graves' hyperthyroidism

OBJECTIVE

A relevant biological role of circulating endothelial progenitor cells (EPC) was recently demonstrated. EPC are generated in the bone marrow, and interact with damaged endothelium, restoring the integrity of the monolayer. Therefore, aim of the present study was to evaluate EPC in the blood of patients with untreated Graves' hyperthyroidism (GD), in whom an increased oxidative stress was observed.

DESIGN AND METHODS

Twenty-three patients with untreated active GD and 18 matched normal controls (NC) were included in the study. Circulating EPC were isolated from peripheral blood. Mononuclear cells were cultured with endothelial basal medium supplemented with EGM SingleQuots, and were identified by positive double staining after 7 days in culture. Circulating levels of C reactive protein, total antioxidant power, interleukin (IL)-6, IL- 18, monocyte chemoattractant protein-1, tumor necrosis facotr- α, soluble vascular cell adhesion molecule (VCAM) and intracellular adhesion molecule were evaluated by enzymelinked immunosorbent assay kit. EPC number was also evaluated in a subgroup of GD patients after restoration of euthyroidism.

RESULTS

Systolic blood pressure resulted increased in GD patients compared with control subjects whereas diastolic blood pressure was not significantly different. Patients with GD showed an increase in circulating levels of IL-18 and VCAM-1 and a reduction of total antioxidant power (p<0.05) compared to NC. Moreover, a reduced number of EPC was observed in patients with GD compared to NC (p<0.05) which turned to NC values after restoring euthyroidism.

CONCLUSION

Patients with GD showed a reduction in the physiological protective mechanisms against endothelial damage, probably induced by increased inflammation and oxidative stress.

Human umbilical cord blood endothelial progenitor cells decrease vein graft neointimal hyperplasia in SCID mice

AIMS

Vein graft endothelial damage is a key step in the development of neointimal hyperplasia, leading to vein graft failure. We sought to determine whether exogenous endothelial progenitor cells could promote vein graft re-endothelialization, and thereby ameliorate neointimal hyperplasia.

METHODS AND RESULTS

Carotid artery interposition grafting was performed with syngeneic inferior vena cavae in mice with severe combined immunodeficiency (SCID). Lineage-negative human umbilical cord blood (hUCB) cells (or medium alone) were injected into vein-grafted mice intra-operatively and 2 weeks post-operatively. In vein grafts from hUCB cell-injected mice, we found human HLA-expressing endothelial cells, as well as increased levels of VEGF and FGF-2. Furthermore, hUCB cells secreted VEGF and FGF-2 in vitro. The markedly enhanced endothelial regeneration, likely resulting from both direct engraftment and paracrine actions of hUCB cells, inhibited inflammatory response, diminished intimal cell proliferation, and reduced neointimal hyperplasia in the vein grafts.

CONCLUSIONS

hUCB cells may accelerate vein graft re-endothelialization via both direct differentiation into endothelial cells and release of paracrine factors to enhance endothelial regeneration and reduce inflammation. These data highlight a potential therapeutic role for cellular therapy in vessel injury.

From bone marrow to the arterial wall: the ongoing tale of endothelial progenitor cells

Several physiological and pathophysiological stimuli or drugs modulate endothelial progenitor cell (EPC) mobilization. Moreover, levels of circulating EPCs predict cardiovascular risk and left ventricular remodelling after myocardial infarction. Nevertheless, our understanding in this field is complicated by lack of an unequivocal definition of EPCs, thus limiting their clinical applications. This review summarizes current knowledge and uncertainties on EPC characterization and mobilization in the attempt to define their role in the management of cardiovascular diseases.

Endothelial progenitor cells dysfunction and senescence: contribution to oxidative stress

The identification of endothelial progenitor cells (EPCs) has led to a significant paradigm in the field of vascular biology and opened a door to the development of new therapeutic approaches. Based on the current evidence, it appears that EPCs may make both direct contribution to neovascularization and indirectly promote the angiogenic function of local endothelial cells via secretion of angiogenic factors. This concept of arterial wall repair mediated by bone marrow (BM)-derived EPCs provided an alternative to the local "response to injury hypothesis" for development of atherosclerotic inflammation. Increased oxidant stress has been proposed as a molecular mechanism for endothelial dysfunction, in part by reducing nitric oxide (NO) bioavailability. EPCs function may also be highly dependent on a well-controlled oxidant stress because EPCs NO bioavailability (which is highly sensitive to oxidant stress) is critical for their in vivo function. The critical question is whether oxidant damage directly leads to an impairment in EPCs function. It was revealed that activation of angiotensin II (Ang II) type 1 receptor stimulates nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase in the vascular endothelium and leads to production of reactive oxygen species. We observed that Ang II accelerates both BM- and peripheral blood (PB)-derived EPCs senescence by a gp91phox-mediated increase of oxidative stress, resulting in EPCs dysfunction. Consistently, both Ang II receptor 1 blockers (ARBs) and angiotensin converting enzyme (ACE) inhibitors have been reported to increase the number of EPCs in patients with cardiovascular disease. In this review, we describe current understanding of the contributions of oxidative stress in cardiovascular disease, focusing on the potential mechanisms of EPCs senescence.

Pioglitazone inhibits angiotensin II-induced senescence of endothelial progenitor cell

We investigated whether a peroxisome proliferator-activated receptor (PPAR) agonist would effect the angiotensin II (Ang II)-induced senescence of endothelial progenitor cells (EPCs). EPCs were isolated from peripheral blood and characterized. Both reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting were used to assess gp91phox expression and angiotensin type 1 receptor (AT1R) levels. Immunofluorescence of nitrotyrosine provided evidence of peroxynitrite formation. Our data indicate that Ang II increased the expression of gp91phox mRNA, which was significantly diminished by pioglitazone, a PPARgamma agonist. Western blotting revealed that Ang II stimulated an increase in the gp91phox protein, whereas co-treatment with pioglitazone significantly reduced this increase. In addition, pioglitazone also inhibited Ang II-induced peroxynitrite formation. Interestingly, pioglitazone decreased the expressions of AT1R mRNA and protein. The exposure of cultured EPCs to Ang II (100 nmol/L) significantly accelerated the rate of senescence compared to that of the control cells during 14 d in culture, as determined by acidic beta-galactosidase staining. Ang II-induced EPC senescence was significantly inhibited by co-treatment with pioglitazone. Because cellular senescence is critically influenced by telomerase, which elongates telomeres, we also measured telomerase activity by means of PCR-ELISA-based assay. The results showed that Ang II significantly diminished telomerase activity, and this effect was significantly abolished by co-treatment with pioglitazone. In conclusion, pioglitazone inhibited Ang II-induced senescence of EPCs via down-regulation of the expression of AT1R.

Endothelial progenitor cells: a primer for vascular surgeons

Endothelial progenitor cells (EPCs) participate in vascular healing during both acute injury and chronic disease. The quantity and quality of circulating EPCs correlate inversely with the severity of vascular disease, such that reduced number and/or function of EPCs are significant independent risk factors for impaired healing capacity, dysfunctional endothelium, and progression of atherosclerosis and vascular disease. EPC therapy assists healing of cardiac and limb ischemia and has great potential for improving the quality of life and longevity of patients with severe cardiovascular and peripheral vascular disease who are not candidates for conventional revascularization procedures. In addition, EPCs can be used to promote vascular graft patency. This review focuses on the characterization of EPCs, positive and negative regulators of EPCs, the role of EPCs in vascular disease, and the potential for EPC therapy to ameliorate the sequelae of severe peripheral vascular disease.

Endothelial outgrowth cells are not derived from CD133+ cells or CD45+ hematopoietic precursors

OBJECTIVE

Two types of endothelial progenitor cells (EPCs), early EPCs and late EPCs (also called endothelial outgrowth cells [EOCs]), were described in vitro previously. In this report, we dissect the phenotype of the precursor(s) that generate these cell types with focus on the markers CD34, CD133, and vascular endothelial growth factor receptor-2 (VEGFR2) that have been used to identify putative circulating endothelial precursors. We also included CD45 in the analysis to assess the relation between CD34+ hematopoietic progenitors (HPC), CD34+ endothelial precursors, and both in vitro generated EPC types. Addressing this issue might lead to a better understanding of the lineage and phenotype of the precursor(s) that give rise to both cell types in vitro and may contribute to a consensus on their flowcytometric enumeration.

METHODS AND RESULTS

Using cell sorting of human cord blood (UCB) and bone marrow (BM) cells, we demonstrate that EOC generating precursors are confined to a small CD34+ CD45- cell fraction, but not to the CD34+ CD45+ HPC fraction, nor any other CD45+ subpopulation. CD34+ CD45+ HPC generated monocytic cells that displayed characteristics typical for early EPCs. Phenotypic analysis showed that EOC generating CD34+ CD45- cells express VEGFR2 but not CD133, whereas CD34+ CD45+ HPC express CD133 as expected, but not VEGFR2.

CONCLUSION

EOCs are not derived from CD133+ cells or CD45+ hematopoietic precursors.

CC chemokine receptor 5 influences late-stage atherosclerosis

Members of the chemokine system, play a central role in inflammatory processes that underlie the pathogenesis of atherosclerosis and possibly, aortic valve sclerosis. Here we show that genetic inactivation of CC chemokine receptor 5 (CCR5) in the atherosclerosis-prone Apoe-/- mice (Apoe-/- Ccr5-/-) fed a normal chow or a high-fat diet (HFD) are protected against advanced atherosclerosis as well as age-associated aortic valve thickening (AAAVT)--a murine correlate of aortic valve sclerosis. Notably, human sclerotic valves contained CCR5+ cells. We confirm that Apoe-/- Ccr5-/- mice does not influence early-atherosclerotic stage. Adoptive transfer studies showed that the atheroprotective effect of CCR5 inactivation resided in the bone marrow compartment, but was not dependent on T-cells. The CCR5-null state was associated with phenotypes postulated to be atheroprotective such as reduced macrophage accumulation in the plaque, and lower circulating levels of IL-6 and MCP-5. The lack of CCR5 expression in Apoe-/- mice was also associated with higher numbers of endothelial progenitor cells (EPCs)--another postulated athero-protective factor. Compared with controls, carriers of a polymorphism in the Ccr5 gene that leads to the lack of CCR5 in the cell surface had an increased mean percentage of EPCs, but this difference did not reach statistical significance. Collectively, these findings underscore a critical role of CCR5 in age-associated cardiovascular diseases, and highlight that the effects of the chemokine system can be temporally constrained to distinct stages of these disease processes.

Aging in the atherosclerosis milieu may accelerate the consumption of bone marrow endothelial progenitor cells

OBJECTIVE

We have demonstrated that bone marrow cells from young and wild-type (WT), but not old apoE-/-, mice are capable of preventing atherosclerosis. This study was performed to elucidate the numerical and functional changes underlying the efficacy difference between young and old bone marrow.

METHODS AND RESULTS

CD34+/VEGFR2+ conventional endothelial progenitor cells and lin-/cKit+/Sca-1+ hematopoietic stem cells did not differ numerically or functionally between young and old apoE-/- bone marrow. Fluorescence-activated cell sorter analysis, however, showed that a group of cells (simple little cells or SLCs), characteristically located in the lower left quadrant of forward scatter/side scatter flow cytometric plot, were markedly decreased in old WT and apoE-/- marrow, but abundantly present in young WT and apoE-/- bone marrow. The SLC fraction was mainly composed of lin-/cKit-/Sca-1- cells. In vitro differentiation assay demonstrated substantially more efficient endothelial differentiation of lin-/cKit-/Sca-1- SLCs than other bone marrow fractions at a single cell level and en masse. Furthermore, old lin-/cKit-/Sca-1- SLCs had a trend of decreased endothelial differentiation capability.

CONCLUSIONS

Lin-/cKit-/Sca-1- SLCs may represent a previously unrecognized cell population, enriched for endothelial progenitors. The identification of these cells may help improve the efficacy of cell therapy.